Single Step Milling and Surface Coating Process for Preparing Stable Nanodispersions

ABSTRACT

A single step milling and surface coating process allows for production of a stable dispersion of surface coated nanoparticles in an efficient manner. The process comprises providing feed particles, providing a coating agent, and generating the stable dispersion of surface coated nanoparticles by milling the feed particles in an aqueous medium containing the coating agent such that the coating agent bonds to surfaces of the feed particles as the feed particles are milled to an average particle size of less than about 100 nm.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/267,400 filed Nov. 7, 2008, which claims priority of U.S. ProvisionalPatent Application Ser. No. 61/020,603 filed Jan. 11, 2008.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND

1. Field of Invention

The present invention relates generally to improved methods forproducing stable dispersions of nanoparticles and, more particularly,but not by way of limitation, to a single step milling and surfacecoating process for producing a stable dispersion of surface coatedmetal oxide nanoparticles.

2. Background of the Invention

Inorganic oxides such as titanium oxide and zinc oxide are oftenincorporated in cosmetics, paint, and plastics as whiteners, opacifiers,tinting agents, and the like. Particles of titanium dioxide, zinc oxide,and the like can also be used as an anti-UV agent in numerousapplications, particularly in cosmetics, paint and plastics.

When used as a pigment, the performance of the particles involvesabsorption, reflection and scattering of visible light, which depends inlarge part on the particle size. For opacification, the optimum particlesize of titanium dioxide is about 250 nm. When particles of titaniumdioxide, zinc oxide, and the like are used as anti-UV agents, theperformance involves absorption, reflection and scattering of theharmful UV radiation, and again depends to a large extent on theparticle size. For example, titanium dioxide absorbs light with awavelength of 405 nm or shorter. However, titanium dioxide also has avery high refractive index and is thus very effective in scattering.There is evidence that submicron titanium dioxide attenuates UVB (with awavelength of from 290 to 320 nm) predominantly by absorption, while UVA(with a wavelength of from 320 to 400 nm) is attenuated predominantly byscattering.

There is a need for particle dispersions which are completelytransparent, free from whitening when applied on the skin, but whichstill possess UV screening capabilities. It is known that when particlesare smaller than one-half the wavelength of visible light, the particleswill appear to form a transparent solution when completely dispersed.Thus, as anti-UV agents, the particles of titanium dioxide, zinc oxide,and the like desirably comprise a stable dispersion of nanoparticles.Nanoparticles generally refer to particles having at least one dimensionof about 100 nanometers or less. Nanoparticles, unlike pigment sizeparticles, scatter UVB light and UVA light more than the longer, visiblewavelengths, and can thus prevent sunburn while remaining transparent onthe skin. However, prior art processes for producing the nanoparticlesare typically quite expensive and the dispersions produced are notstable.

For example, pigment particles are produced in a high-temperaturereactor and then surface treated with metal silicates, dried, andfurther micronized to reduce particle agglomeration. Similar processesare available for producing nanoparticles which can potentially offerthe desired transparency. For instance, the synthesis of nanoparticlesof metals and mixed metal oxides through high-temperature oxidation ofreactive precursors in oxygen plasma has been known for some time.However, plasma processing leads to agglomeration of the nanoparticles,which detracts from their desirability, especially where nanoparticleperformance is required. Additionally, the high costs associated withplasma processing leads to costly end products and further limits theircommercial attractiveness.

Ultra fine grinding techniques have also been investigated. As theparticles become very small, their total surface area and surface energybecome quite large, resulting in very high stress requirements forfurther fracture of the particles. Traditionally, a s the particlesdecrease in size, the particles begin to flocculate or coagulate inorder to decrease the total surface energy. Eventually particle sizereduction approaches a limit and maximum energy is expended. Thus thereis a need for lower cost, lower energy processes for producing metaloxide nanoparticles without aggregation and agglomeration.

Because titanium dioxide and zinc oxide are photoactive, i.e., freeradical generators, to be effective in ultraviolet (UV) attenuationapplications it is desirable to surface treat the titanium dioxide andzinc oxide nanoparticles to minimize or eliminate such activity.Particle absorption of a photon can result in production of a hole andan electron which can migrate to the surface of the particle and, inaqueous environments, form superoxide and hydroxyl radicals. It is knownthat a coating can capture these radicals and thereby reduce theapparent photoactivity.

Known processes for producing coated titanium dioxide particlestypically include wet processing of particles that have been formed byplasma processing, ultrafine grinding or precipitation. During wetprocessing the particles are filtered, an aqueous slurry of the titaniumdioxide particles is prepared, and the slurry is then treated with ametal precursor or salt to precipitate a metal oxide or hydroxide on theparticle surfaces. Generally these surface coatings tend to causeadditional agglomeration of the particles such that the coated titaniamust again be filtered, dried, re-ground to reduce agglomeration, andthen redispersed. Although these coating methods can be used, themethods involve multiple energy-intensive steps.

Thus there are continuing needs for improved processes for making andcoating metal oxide nanoparticles and producing stable dispersionstherefrom.

SUMMARY OF THE INVENTION

One embodiment of the present invention is directed to a single stepmilling and surface coating process that allows for production of astable dispersion of surface coated nanoparticles in an efficientmanner. The process includes combining feed particles comprising zincoxide having an average primary particle size of 200 nm or greater withan aqueous medium containing a coating agent. The stable dispersion ofsurface coated nanoparticles is generated by milling the feed particlesat a concentration of 10% to 75% by weight in the aqueous mediumcontaining the coating agent, such that the coating agent bonds tosurfaces of the feed particles as the feed particles are milled to anaverage particle size of 100 nm or less.

In another embodiment, a method for generating a stable dispersion ofsurface coated nanoparticles is provided utilizing the followingprocedure. Feed particles comprising zinc oxide having an averageparticle size of 200 nm or greater are combined with a coating agent ina disperser to provide a slurry having a solids content of about 35% orless by weight of slurry. The slurry is circulated to an agitated mediamill in closed loop with the disperser such that at least a portion ofthe coating agent bonds to surfaces of the feed particles as the feedparticles are milled. An additional amount of feed particles is suppliedto the slurry, the additional amount sufficient to increase the solidscontent of the slurry to a range of from about 35% to about 75% byweight of slurry. The slurry having a solids content in the range offrom about 35% to about 75% by weight is circulated to the agitatedmedia mill in closed loop with the disperser such that the coating agentbonds to surfaces of the feed particles as the feed particles are milledto an average particle size of 100 nm or less.

Thus, utilizing (1) the technology known in the art; (2) theabove-referenced general description of the presently claimed and/ordisclosed inventive process(es), methodology(ies), apparatus(es) andcomposition(s); and (3) the detailed description of the invention thatfollows, the advantages and novelties of the presently claimed and/ordisclosed inventive process(es), methodology(ies), apparatus(es) andcomposition(s) would be readily apparent to one of ordinary skill in theart.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical representation of the particle size distributionof a nanodispersion produced using the procedure described in Example 2of the present disclosure.

FIG. 2 is a graphical representation of the particle size distributionof a nanodispersion produced using the procedure described in Example 3of the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION

Before explaining at least one embodiment of the invention in detail, itis to be understood that the invention is not limited in its applicationto the details of construction, experiments, exemplary data, and/or thearrangement of the components set forth in the following description.The invention is capable of other embodiments or of being practiced orcarried out in various ways. Also, it is to be understood that theterminology employed herein is for purpose of description and should notbe regarded as limiting.

As mentioned above, many applications for metal oxides require stabledispersions of nanosize coated metal oxide particles. The presentinvention provides a single step milling and surface coating process forproducing a stable dispersion of surface coated nanoparticles, thusavoiding the multiple energy intensive steps used in much of the priorart. Feed particles and a coating agent are milled together in anaqueous medium such that the coating agent bonds to surfaces of the feedparticles as the feed particles are milled to nanoparticles. The term“nanoparticles” as used herein refers to particles having at least onedimension of about 100 nanometers or less.

Any metal oxide feed particle can be used. Suitable feed particles forUV-screening applications include, but are not limited to, titaniumdioxide, zinc oxide, zinc titanate and iron oxide. In one embodiment,the feed particles comprise titanium dioxide. In another embodiment, thefeed particles comprise a mixture of titanium dioxide and zinc oxide.Feed particles generally have an average particle size in a range offrom about 100 nm to about 2500 nm prior to milling. For example, feedparticles comprising metal oxides, such as titanium and zinc oxides, aregenerally pigment grade or agglomerated forms where the particle sizerange can extend from 150 nm to 1500 nm and greater.

Titanium dioxide feed particles can be either anatase or rutile, both ofwhich are readily available commercially as, for example, pigments.Anatase is produced by a process commonly known as the sulfate process.Illmenite is dissolved in sulfuric acid and the by-product iron sulfateis crystallized out of solution. The dissolved titanium is furtherpurified and precipitated to yield anatase titanium dioxide particles.

Rutile is produced using the chloride process wherein crude titaniumdioxide is purified via titanium tetrachloride. The ore or enriched oreis carbochlorinated with carbon and chlorine to give a titaniumtetrachloride vapor. This titanium tetrachloride is purified andre-oxidized with oxygen to give predominantly rutile titanium dioxide.

Preferably, the titanium dioxide crystal structure is predominantlyanatase; however, a rutile titanium dioxide crystal structure andmixtures of anatase and rutile can also be used. In certainapplications, such as cosmetics and dermal care products, the rutileform (a hard crystal) is less desirable than the anatase form (a softercrystal). When pigment is used as the feed material, the feed particlestypically have an average primary particle size of about 0.2 micron (200nm) or greater to optimize the light scattering capability of thepigment particles and are typically agglomerated to form much largeragglomerates.

Zinc oxide is a base white pigment used in paints, cosmetics, and as anopaque sunscreen. It is also used in the rubber industry, electronicmaterials and medical applications. Zinc oxide can be manufactured bygas phase chemical deposition, spray pyrolysis and sol gel methods. TheFrench process for manufacture of zinc oxide involves melting metalliczinc in a graphite crucible and vaporizing the zinc metal at hightemperatures. The zinc vapor reacts with oxygen to produce zinc oxideparticles which are cooled and collected in a bag house.

In one embodiment of the present invention, the feed particles comprisea mixture of anatase and zinc oxide. Milling mixtures of anatase andzinc oxide appears to have a synergistic effect on the photocatalyticproperties of the particles. Surprisingly, and as described in moredetail hereinafter, a mixture of anatase and zinc oxide milled andcoated together according to an embodiment of this invention, results ina reduced photocatalytic activity compared to a mixture having the samechemical composition but not milled and coated together. When titaniumdioxide and zinc oxide are milled together, the weight ratio of titaniumdioxide to zinc oxide is preferably in a range of from about 1:10 toabout 10:1, and more preferably in a range of from about 2:1 to about1:2.

An aqueous medium and the feed particles to be milled are firstcombined, preferably in a disperser or high-shear mixer, to form aslurry. The terms “disperser” and “high-shear mixer” are usedinterchangeably herein and mixing time can extend up to three to fourhours. The aqueous medium comprises water, wherein the term “water” isused includes groundwater, sea water, and brines as well as treatedwater such as deionized or distilled water. Preferably, the water isdeionized or distill ed. Feed particles are typically added to theaqueous medium to form a slurry having a wt % solids in the range offrom about 10 to about 40 wt %, and preferably in the range of fromabout 25 to about 30 wt %. The slurry pH is monitored and adjusted with,for example, aminomethyl propanol (AMP-95™) to maintain a minimum basicpH level of about 9.0. Additional feed particles can be added toincrease the total solids content to a range of from about 50% to about70 wt %.

As discussed previously, oxides such as titanium dioxide and zinc oxidecan provide excellent UV protection; however, a disadvantage is theirphotocatalytic activity. By generating free radicals, the particles canaccelerate degradation of constituents in the carrier composition.Photocatalytic activity is reduced by covering the particle surfaceswith a coating or coating agents. Prior art coating procedures involveadding coating agents to the particles in a separate step after theparticles have been reduced to the desired size. Aggregation due to thecoating process is then reduced by milling the coated aggregates in yetanother separate step. However, it has presently been discovered thatthe particle surfaces can be coated while the particles are being milledsuch that the coating agent binds to the surface of the feed particlesand to the newly formed surfaces of the feed particles during milling toprovide a surface coating on the nanoparticles. It has been found thatsuch a single step milling and coating process synergistically improvescoating efficiency and coated particle properties.

Coating agents can be organic as well as inorganic compounds, or acombination of both, wherein the compounds are capable of bonding to thesurface of the oxide particle. Examples of suitable organic compoundsinclude, but are not limited to acrylate polymers, organosiliconcopolymers and organosilicon compounds. Preferably, a polyacrylatepolymer such as ammonium polyacrylate is used. Ammonium polyacrylate isavailable commercially as, for example, Darvan® from R.T. VanderbiltCompany, Inc. of Norwalk, Conn. This results in an organic surfacetreatment that incorporates new performance properties in the finalnanodispersion. The optimum amount of organic coating agent varies, butis typically in a range of from about 5% to about 25% of the total feedparticle weight. Preferably the amount of organic coating agent added isin a range of from about 5% to about 10% of the total feed particleweight. The coating can be added to the disperser or directly to themill.

Many inorganic coating agents can be utilized. For example, solublemetal compounds can be added such that at least one metal oxide,hydroxide, or hydrated oxide binds to the surface of the feed particlesand to the newly formed surfaces of the feed particles during milling toprovide a surface coating on the surface coated nanoparticles. Suitablemetal compounds include, but are not limited to, soluble salts, oxidesand hydroxides o f aluminum, zinc, calcium, tin, iron and zirconium. Inone embodiment, the soluble metal compound may preferably be sodiumaluminate. A silicate such as sodium silicate or silicic acid, and metalsilicates such as aluminum silicate may also be used as coating agents.In this case the silicate binds to the surfaces of the feed particles asthe feed particles are milled to nanoparticles.

The amount of inorganic coating agent varies but is typically in a rangeof from about 0.5% to about 20% of the total feed particle weight.Preferably, the amount of inorganic coating agent added is in a range offrom about 0.5% to about 5% of the total feed particle weight.

Because the coating process occurs while milling, insoluble coatingcompounds can be added such that the surfaces of the coating compoundcan adhere to or react with the surfaces of the feed particles duringcompaction. For example, milling a mixture of zinc oxide and titaniumdioxide together can produce nanoparticles with a reduced photocatalyticactivity compared to a mixture having the same chemical composition butnot milled and coated together. While not limiting the invention to anyparticular theoretical mechanism, it is believed that compaction of atitanium dioxide surface with a zinc oxide surface can result information of a zinc/titanium compound such as zinc titanate on thesurfaces in a manner similar to mechanical alloying processes used withelemental powders.

Coating of the particles generally improves the photocatalytic activityof the particles. Photocatalytic activity is typically measured relativeto a standard. For example, the sample can be suspended in a 0.40Msolution of 2-propanol in pentane and irradiated with UV light for aspecified time. The concentration of acetone formed is measured andcompared to the acetone formed from a standard under the sameconditions. Conversion of 2-propanol is determined by gaschromatography. Acetone concentration is typically used to measurephotocatalytic activity because the rate of formation follows a zeroorder kinetics model.

Typically a dispersant such as an organic acid is added to the slurryprior to milling. The dispersant aids in the creation of ananodispersion as the milling process proceeds and the feed particlesare reduced in size. The dispersant used in this invention can be anydispersant known in the art to be useful in the fine grinding of metaloxides and includes, but is not limited to, citric acid, sodium orpotassium pyrophosphate, aliphatic carboxilic acids, polyhydroxyalcohols, triethanol amine (TEA), and 2-amino 2-methyl 1-propanol(AMP™), monoisopropylamine, and mixtures thereof. A preferred organicacid dispersant is citric acid.

The amount of organic acid dispersant added can be in the range of fromabout 1% to about 20% based on the weight of feed particles. Preferablyabout 10% dispersant based on the weight of feed particles is added. Itis common practice to add all of the dispersant to the slurry in thedisperser at the start of milling. In some instances, however, it isadvantageous to add the dispersant gradually during milling in amountsto ensure minimal flocculation of the feed particles during milling.

Good dispersion is beneficial to enable milling to the nanoparticle sizerange. However, long term dispersion is additionally required when thenanoparticles are used in applications such as sunscreens and cosmetics.It is known to add certain dispersing agents to coated titania particlesto produce a nanoparticle suspension; however, prior art suspensions donot typically remain stable for long periods of time. In suchapplications, reagglomeration of the nanoparticles over time results ina “rough” feeling when spread onto the skin and general aestheticallyundesirable characteristics.

Upon preparation of the desired slurry composition, the resulting feedslurry is charged to an attritor or agitated media mill to begin thesize reduction process. Suitable agitated media mills are known to thoseskilled in the art. For example, one such suitable mill is a bead millmanufactured by and available from Netzsch, Inc. The North Americansubsidiary of the Netzsch operating companies is located in Exton, Pa.

The agitated media mill, sometimes referred to herein as a bead mill, isemployed to shear or reduce the feed particles to the nanoscale (<100nm) range from a large starting size that can be microns in range. Themill is charged with the appropriate amount of media, typically 70 to 95vol %, preferably 85 to 95 vol %, and the remaining mill volume ischarged with feed slurry from the disperser. Under the appropriateconditions of milling media, temperature, flow and mill rotation, thefeed particles can be rapidly reduced to nanoscale size.

The milling media is comprised of dense beads of materials such asyttria stabilized zirconia in a uniform size range to achieve desiredproduct particle size ranges. For example, a milling media composed of100 to 200 micron zirconia will generally produce a limiting productsize of approximately 100 nm. A milling media composed of 50 to 100micron zirconia will similarly produce a limiting product size ofapproximately 50 nm. Preferably the milling media is zirconia having aparticle size less than 200 microns. In some instances it is desirableto use zirconia milling media with a particle size in the range of fromabout 30 microns to about 50 microns. Of course, the milling media alonedoes not lead to stable nanodispersions.

The energy expended in milling causes a significant temperature increasein the mill. As understood by those skilled in the art, temperature canbe controlled by a number of means, such as by utilizing cooling waterwith a jacketed mill or by circulating the slurries through a heatexchanger with cooling water. A heat exchanger can also be disposedwithin the disperser. Preferably, the temperature is maintained at 45°C. or less inside the mill to avoid damage to temperature-sensitiveseals and the like.

The milling process is monitored and controlled to produce the desiredsurface area or particle diameter. BET surface area is understood tomean the specific surface area determined by nitrogen adsorption inaccordance with ASTM standards. Particle size can also be determined bytransmission electron microscopy (TEM) and by particle size distributionas determined by a particle size analyzer using dynamic lightscattering. When the surface area is calculated based on the particlesize distribution measured by dynamic light scattering, the specificsurface area computation typically assumes smooth, solid, sphericalparticles as opposed to a BET or other adsorption based surface areadetermination.

The milling process is preferably monitored by a particle size analyzer,such as the NanoTrak available from Micro-Trak Systems, Inc. of EagleLake, Minn. Samples from the milling volume are taken periodically andmeasured for volume average and number average particle diameter andspecific surface area.

As the nanomaterial specific surface area increases during the run, froman initial 10 to 20 m²/cm³ (for roughly 1-5 micron diameter feedparticles) to greater than 200 m²/cm³ (for roughly 30nm diameterparticles), the appearance of the slurry changes from a whitedispersion, from which oxide particles will precipitate or settle, to anopaque or translucent nanodispersion, from which no particles willprecipitate or settle. This transition occurs around 150 m²/cm³ orroughly 50nm diameter particles. The process can be terminated at thispoint or continued to produce higher surface area materials. The coatednanosize product at this point is referred to as an intermediate and canbe used as the base for preparing coating or cosmetic formulations.

In one embodiment, the feed slurry is initially subjected to dispersion,sometimes referred to herein as high-shear mixing, for a period ofhours, during which the slurry pH is monitored and adjusted with, forexample, aminomethyl propanol (AMP-95™) to maintain a minimum basic pHlevel of about 9.0. The terms “dispersion” and “high-shear mixing” areused herein and in the appended claims to mean mixing with sufficientshear to minimize particle agglomeration and particle flocculation. Asunderstood by those skilled in the art, the actual disperser design andoperation can vary with feed source, solids loading, slurry chemistry,and other operating conditions. Slurry from the disperser is fed to anagitated media mill in closed loop with the disperser such that at leasta portion of the coating agent bonds to surfaces of the feed particlesas the feed particles are milled. Particle agglomeration andflocculation are minimized by alternating high shear mixing with highshear milling. The slurry is recirculated until the feed particles aremilled to an average particle size of less than about 100 nm.

In another embodiment, feed particles are dispersed and combined with acoating agent in a disperser to provide a slurry having a solids contentless than about 35% by weight of slurry. The slurry is circulated to anagitated media mill in closed loop with the disperser such that at leasta portion of the coating agent bonds to surfaces of the feed particlesas the feed particles are milled. After milling for a time, anadditional amount of feed particles are then supplied to the slurry tobring the solids content to a range of from about 45% to about 75% byweight of slurry. Staging the addition of feed particles allowspreparation of a nanoparticle dispersion with higher solids content thanwould otherwise be possible. Staging also allows for broadening theparticle size distribution if desired for UVA and UVB attenuation.

The process technology can be further extended to the specificpreparation of various finished product formulations. For example, inthe preparation of cosmetic or skin care formulations, the formulationingredients or components are added to the mill and processed using 100micron milling media. The formulation is processed to a predeterminedend point of viscosity and consistency and can be moved directly tofilling stages.

In order to further illustrate the present invention, the followingexamples are given. However, it is to be understood that the examplesare for illustrative purposes only and are not to be construed aslimiting the scope of the invention.

EXAMPLE 1

In this experiment, a 0.5 L MiniCer laboratory grinding system(available commercially from Netzsch, Inc.) is used. Feed particles ofcommercially available anatase pigment having an average primaryparticle size of 0.25 micron are mixed with deionized water and reagentsto form about 350 mL of slurry comprising 15 wt % anatase, 10 wt %citric acid, and 5 wt % Darvan®7-N (ammonium polyacrylate). The pH isadjusted to 9.2 with aminomethyl propanol (AMP-95™). The slurry is mixedfor 5 hours in a disperser using a high shear mixer. The mill is filledwith about 150 mL of YTZ® grinding media (yttrium stabilized zirconia)having a particle size of 0.05 mm. The dispersed slurry is then fedcontinuously through the mill and recirculated through the disperser.The disperser is water cooled to maintain a temperature between 20° C.to 30° C.

The feed particles are milled for 30 minutes after which a sample isremoved and analyzed for particle size distribution using a NanoTrak™particle size analyzer. The feed particles are milled for another 30minutes after which another sample is removed and analyzed for particlesize distribution. Milling and sampling continue for 6.5 hours. T able 1shows the average particle size achieved at each 30 minute samplingpoint. As understood by those skilled in the art, the term “MV” refersto the mean diameter, in nanometers, of the volume distribution andrepresents the center of gravity of that distribution. The averageparticle size based on the volume distribution is strongly influenced bycoarse particles. The term “MN” refers to the mean diameter, innanometers, of the number distribution and is weighted to the smallerparticles. The term “CS” refers to the calculated specific surface areain m²/cm³. The computation assumes smooth, solid, spherical particles asopposed to a BET or other adsorption based surface area determination.

TABLE 1 Particle Size versus Milling Time Time MV MN CS (min) (nm) (nm)(m²/cm³) 30 96.2 65.1 71.99 60 169.9 82.5 44.2 90 159.9 75 46.61 120128.6 37.6 56.46 150 107.9 43.6 72.84 180 59 29.13 139.6 210 38.4 25.58191.3 240 30.8 23.01 218.1 270 27.73 21.57 240.1 300 25.12 19.92 259.8330 23.54 18.9 275.5 360 22.78 17.86 290.3 390 22.63 17.88 296.8

EXAMPLE 2

A 10-liter bead mill from Netzch, Inc. is used to mill a commerciallyavailable anatase pigment having an average primary particle size ofabout 0.25 micron and an agglomerated particle size of about 1.5 micron.The pigment is mixed with deionized water and reagents to form about 60gal of slurry comprising about 45 wt % anatase, and 5 wt % reagents.Reagents include citric acid and Darvan®7-N. The pH is adjusted to about9 with aminomethyl propanol (AMP-95™). The slurry is dispersed for about4 hours in an 80 gal disperser equipped with a high shear mixer. Themill is about 90 vol % charged with YTZ® grinding media having aparticle size in the range of 100 to 200 micron and finer. The dispersedslurry is then fed continuously through the mill and recirculatedthrough the disperser while maintaining the temperature at less thanabout 45° C. to avoid damage to synthetic materials in the mill. After5-6 hours, surface areas exceeding 500 m²/cm³ are achieved and theresulting dispersion remains stable for at least two weeks and typicallyfor greater than two months. FIG. 1 shows a typical product particlesize distribution.

EXAMPLE 3

The procedure described in Example 2 is modified such that the slurryinitially contains only a portion of the anatase feed particles andreagents in a 25% solids slurry. The initial slurry is dispersed for 1-2hours and then circulated through the mill in closed loop with thedisperser for several additional hours, after which the remaininganatase feed particles and reagents are added to the slurry. Theresulting slurry contains 55% solids by weight and is circulated throughthe mill for several additional hours giving a total milling anddispersion time of about 5-6 hours.

FIG. 2 shows a product particle size distribution using the aboveprocedure. The broad nature of the particle size distribution is morepronounced. Such a broad distribution can be very desirable forscreening both UVA and UVB in sun screen formulations and applications.In addition, it is possible to produce a stable and very high solidsdispersion of titania nanoparticles.

EXAMPLE 4

The photoactivity of an anatase nanodispersion having an organic coatingof Darvan®7-N and produced according to Example 1 was compared toAEROXIDE® P25, a commercially available uncoated nanoparticle titaniafrom Degussa (now Evonik Industries). Sample dispersions of thecommercial grade and the product produced by this invention were placedin contact with polyethylene films and exposed to a period of UVradiation. The product produced by Example 1 of this invention exhibitedno UV effects on the film while the commercial uncoated grade exhibitedmarks and discoloration on the film.

EXAMPLE 5

The procedures of this invention may also be used to treat zinc titanateto produce substantially stable dispersions of nanoparticles. Zinctitanate feed particles having an average agglomerated particle size of1-5 micron are milled using a 0.5 L MiniCer laboratory grinding system.The feed particles are mixed with deionized water and reagents to formabout 350 mL of premix slurry comprising 15 wt % zinc titanate, 10 wt %citric acid, and 5 wt % Darvan®7-N (ammonium polyacrylate). The pH isadjusted to 9.2 with aminomethyl propanol (AMP-95™). The slurry is mixedin the disperser while the mill is filled with about 150 mL of YTZ®grinding media having a particle size in the range of 100 to 200 micron.The dispersed slurry is then fed continuously through the mill andrecirculated through the water cooled disperser while maintaining atemperature of 45° C. The zinc titanate feed particles are milled andcirculated for a total of 5 hours.

EXAMPLE 6

A dispersion of silica-coated titania nanoparticles may be producedusing the following procedures. Feed particles of commercially availableanatase pigment having an average primary particle size of 0.25 micronare mixed with deionized water and reagents to form about 350 mL ofslurry containing about 30 wt % anatase, about 5 wt % citric acid, andabout 8 wt % sodium silicate. The pH is adjusted to 9.0 with eitheraminomethyl propanol (AMP-95™) or additional citric acid depending onthe alkalinity of the sodium silicate. The slurry is dispersed for about1 hour in a disperser using a high shear mixer. The 0.5 L MiniCerlaboratory grinding system is filled with about 150 mL of YTZ® grindingmedia (yttrium stabilized zirconia) having an average particle size ofabout 50 micron. The dispersed slurry is fed continuously through themill and recirculated through the disperser. The mill is jacketed andwater cooled to maintain a temperature less than about 40° C. to avoiddamage to the mill, and the temperature of the slurry in the disperseris controlled to a temperature in the range of from about 45° C. toabout 60° C.

The feed particles are milled for about 3 hours while samples areremoved and analyzed to determine degree of flocculation and primaryparticle size distribution. Darvan®7-N is added to reduce or eliminateflocculation. The pH is monitored and adjusted to provide the desiredsilica coating on the titania nanoparticles.

EXAMPLE 7

A dispersion of alumina-coated titania nanoparticles may be producedusing the following procedures. Feed particles of commercially availableanatase pigment having an average primary particle size of 0.25 micronare mixed with deionized water and reagents to form about 60 gal ofslurry containing about 30 wt % anatase, about 3 wt % citric acid, andabout 8 wt % sodium silicate. The pH is adjusted to 9.0 with eitheraminomethyl propanol (AMP-95™) or additional citric acid, depending onthe alkalinity of the sodium aluminate. The slurry is dispersed forabout 1 hour in a disperser using a high shear mixer. A 10-liter beadmill from Netzch, Inc. is about 90% filled with YTZ® grinding mediahaving an average particle of about 100 micron. The dispersed slurry isthen fed continuously through the mill and recirculated through thedisperser. The disperser is maintained at about 60° C. to 70° C. and themill is jacketed and water cooled to maintain the temperature less thanabout 45° C.

The feed particles are milled for about 3 hours while samples areremoved and analyzed to determine degree of flocculation and primaryparticle size distribution. Darvan®7-N is added to reduce or eliminateflocculation. The pH is monitored and adjusted to provide the desiredalumina coating on the titania nanoparticles.

EXAMPLE 8

Feed particles of commercially available anatase pigment having anaverage primary particle size of 0.22 micron are mixed with zinc oxidepigment having an average primary particle size of 0.23 micron. Thepigments are combined in a 1:1 weight ratio and are mixed with deionizedwater and reagents to form about 350 mL of slurry comprising 15 wt %anatase and zinc oxide, 10 wt % citric acid, and 5 wt % Darvan®7-N(ammonium polyacrylate). The pH is adjusted to 9.2 with aminomethylpropanol (AMP-95™). The slurry is dispersed for 2 hours in a disperserusing a high shear mixer. The mill is filled with about 150 mL of YTZ®grinding media (yttrium stabilized zirconia) having a particle size of0.05 mm. The dispersed slurry is then fed continuously through the milland recirculated through the disperser. Slurry in the mill and disperseris cooled to maintain a temperature of less than about 45° C.

The anatase and zinc oxide feed particles are milled and circulatedthrough the disperser for a total of about 5 hours. The resultingdispersion of anatase and zinc oxide nanoparticles has an averageprimary particle size of about 30 nm. The solids content of theresulting nanodispersion can be increased from about 25-30 wt % to about50 wt % by adding additional feed particles to the disperser aftermilling the lower solids content slurry for about 1 to 2 hours. Thephotocatalytic properties of the resulting dispersions are measured bycoating a clear plastic sheet with a known weight of dispersion,pressing it down, and using a UV meter to measure the total UVA and UVBpenetrating the sheet while exposed perpendicularly to sunlight. The UVpenetrating is compared to the total UV exposure in the sunlight and theresults are expressed as %UVA/UVB blocking efficiency. A 25% solidsnanoparticle dispersion blocks about 85% to 90% of the UVA/UVB radiationwhile a 50% solids dispersion blocks about 95% to about 99% of theUVA/UVB radiation.

EXAMPLE 9

Sunscreen formulations: SPF measurements show that cosmetic formulationscontaining 1 wt % and 5 wt % nanosize titanium dioxide (>150 m²/cm³)provide SPF protection of 15 and 35, respectively. Dermal careformulations are also clear on application, not leaving white residuestypical of larger particle sized titanium dioxide or zinc oxide.

From the above description, it is clear that the present inventiveprocess(es), methodology(ies), apparatus(es) and composition(s) are welladapted to carry out the objects and to attain the advantages mentionedherein as well as those inherent in the presently provided disclosure.While presently preferred embodiments of the invention have beendescribed for purposes of this disclosure, it will be understood thatnumerous changes may be made which will readily suggest themselves tothose skilled in the art and which are accomplished within the spirit ofthe presently claimed and disclosed inventive process(es),methodology(ies), apparatus(es) and composition(s) described herein.

What is claimed is:
 1. A single step milling and surface coating processfor producing a stable dispersion of surface coated nanoparticles, thesingle step process comprising: combining feed particles comprising zincoxide having an average primary particle size of 200 nm or greater withan aqueous medium containing a coating agent; and generating a stableaqueous dispersion of surface coated nanoparticles by milling the feedparticles at a concentration of 10% to 75% by weight in the aqueousmedium containing the coating agent such that the coating agent bonds tosurfaces of the feed particles as the feed particles are milled to anaverage particle size of 100 nm or less.
 2. The process of claim 1,wherein the feed particles additionally comprise titanium dioxideparticles.
 3. The process of claim 1, wherein the feed particles have anaverage primary particle size of from about 200 nm to about 2500 nmprior to milling.
 4. The process of claim 1, wherein the coating agentincludes a polymer or copolymer comprising acrylates or organosiliconcompounds.
 5. The process of claim 1, wherein the coating agentcomprises a polyacrylate polymer.
 6. The process of claim 1, wherein thecoating agent comprises at least one water soluble metal compound thatbinds to surfaces of the feed particles and to newly formed surfaces ofthe feed particles as a metal oxide, hydroxide or hydrous oxide toprovide the surface coating of the surface coated nanoparticles.
 7. Theprocess of claim 6, wherein the at least one water soluble metalcompound comprises a metal selected from the group consisting of zinc,aluminum, zirconium, titanium, iron, calcium or a combination thereof.8. The process of claim 1, wherein the coating agent comprises at leastone water soluble silicate compound such that at least one hydroussilicate or hydrous metal silicate binds to surfaces of the feedparticles and to newly formed surfaces of the feed particles to providethe surface coating of the surface coated nanoparticles.
 9. The processof claim 1, wherein the feed particles are present in the aqueous mediumin an amount in the range of from about 35% to about 75% by weight. 10.The process of claim 1, wherein the aqueous medium additionally containsa dispersant.
 11. The process of claim 10 wherein the dispersantcomprises an organic acid and/or a salt of the organic acid.
 12. Theprocess of claim 10, wherein the dispersant is selected from the groupconsisting of citric acid, polyacrylates, sodium or potassiumpyrophosphate, aliphatic carboxilic acids, polyhydroxy alcohols,triethanol amine, 2-amino 2-methyl 1-propanol triethanolamine,2-amino-2-methyl-1-propanol, monoisopropylamine, and mixtures thereof.13. The process of claim 10, wherein the dispersant comprises citricacid, a salt of citric acid, or a combination of citric acid and a saltof citric acid.
 14. The process of claim 10, wherein the dispersant ispresent in the aqueous medium in an amount in the range of from about 1%to about 20% by weight of aqueous medium.
 15. The process of claim 1,wherein the pH of the aqueous medium is monitored and adjusted.
 16. Theprocess of claim 1, wherein milling and coating the feed particles isachieved in an agitated media mill using a milling media comprisingbeads.
 17. The process of claim 16, wherein the milling media compriseszirconia beads having a particle size less than about 200 nm.
 18. Theprocess of claim 16, wherein the feed particles and aqueous medium arecycled from a disperser to the agitated media mill.
 19. The process ofclaim 1, wherein the dispersion of surface coated nanoparticles remainsin a stable dispersed state for a period greater than about two weeks.20. A process for producing a stable dispersion of nanoparticles, theprocess comprising: combining feed particles with a coating agent in adisperser to provide a slurry having a solids content of about 35% orless by weight of slurry, wherein the feed particles comprise zinc oxidehaving an average particle size of 200 nm or greater; circulating theslurry to an agitated media mill in closed loop with the disperser suchthat at least a portion of the coating agent bonds to surfaces of thefeed particles as the feed particles are milled; supplying an additionalamount of feed particles to the slurry, the additional amount sufficientto increase the solids content of the slurry to a range of from about35% to about 75% by weight of slurry; and circulating the slurry havinga solids content in the range of from about 35% to about 75% by weightof slurry to the agitated media mill in closed loop with the dispersersuch that the coating agent bonds to surfaces of the feed particles asthe feed particles are milled to an average particle size of 100 nm orless.
 21. The process of claim 20, wherein the slurry having a solidscontent of about 35% or less by weight of slurry is milled to an averageparticle size of 100 nm or less prior to supplying the additional amountof feed particles.
 22. The process of claim 20, wherein the coatingagent comprises a polyacrylate polymer.
 23. The process of claim 20,wherein the additional amount of feed particles is sufficient toincrease the solids content of the slurry to a range of from about 45%to about 75% by weight of slurry.